1. Field of the Invention
This invention relates to the treatment of oxidic materials to remove selectively one or more constituents thereof by a chlorination process.
2. Brief Description of the Prior Art
More particular, a fluidised bed chlorination process is envisaged the oxidic materials to be treated usually being a mineral such as an ore, a treated ore, a clay or the like in a particulate form suitable for fluidisation.
One branch of the art of fluidised bed chlorination is that in which an oxidic material is treated with diluted chlorine in the presence of an excess of carbon. In such a process the selectivity of the chlorination is controlled by the dilution of the chlorine, the carbon purposely being removed as a controlling factor by its use in excess over the quantity of oxygen added to the bed, if any, or present in the chlorinatable constituents of the oxidic material in the bed and the temperature may be controlled, if required, by adding a small proportion of oxygen to the bed or by external means of various kinds such as the preheating of the oxidic material.
Such a process as is last described was disclosed in British Pat. No. 1359882 in relation to the beneficiation of an ore containing titanium and iron oxides. Later, in European Patent Publication No. 0034434 a similar type of process was disclosed in relation to the separation of iron and iron chlorides from an oxidic material to leave behind a bed product beneficiated with respect to its content of a desired metal oxide or oxides other than iron and also in relation to the removal of one or more metals other than iron in the form of chlorides, for example, one or more of boron, gallium, zirconium, thorium, aluminium, titanium, silicon or uranium, from an oxidic material either to recover them as desired metals from a residual bed rich in iron oxides or to beneficiate the oxidic material with respect to its content of yet a different metal oxide.
In each aspect of the process disclosed in European Patent Publication No. 0034434 the Gibbs free energy of chlorination of the oxide which is desired to remain in the bed product, is less negative than that removed as chloride from the bed, preferably, though not essentially, at least 10 kilocalories (=10.times.4184 Joules) per mole of chlorine employed less negative.
Reference is made to "The thermochemical properties of the oxides, fluorides and chlorides to 2500.degree. K." by A. Glassner, published by the Argonne National Laboratory, which describes the Gibbs Free Energy of chlorination of metal oxides.
While the type of process discussed above, referred to for brevity as the "dilute chlorine" process can give excellent selectivity of chlorination under some circumstances it is inevitable that a small proportion of the material which it is desired to remove will remain in the bed and contaminate the product drawn from the bed. This effect may be more marked in relation to certain more difficult metal separations, for example in relation to the separation of iron and aluminium values, which is notoriously difficult as is acknowledged in Canadian Pat. No. 488695.
In fluidised bed chlorination processes, generally, it has been desired to minimise chlorine wastage resulting from chlorine slip through the bed by endeavouring to achieve substantially complete utilisation of the chlorine by the time the bed gases reach the upper surface of the bed. Conversely, however, it has been desired to maximise the capacity of the fluidised bed equipment by ensuring that free chlorine is available for reaction in substantially the whole of the bed volume.